A quantum key distribution system is configured with a transmitter, a receiver, and an optical fiber link that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the optical fiber link. Then, the transmitter and the receiver exchange control information with each other, and share a bit string in a confidential fashion for the purpose of generating cryptographic keys. This technology is implemented using the technology generally referred to as quantum key distribution (QKD).
Among such quantum key distribution systems, a system which is configured with a plurality of transmitters, a plurality of optical fiber links formed by branching by an optical device, and a single receiver; and in which the receiver receives photons from a plurality of transmitters via the optical device is called a quantum access network (QAN). As a network that has an identical network configuration to a QAN and that performs optical data communication, a passive optical network (PON) is known. In the optical data communication performed by a PON, there are two types of data, namely, data in the downlink direction (hereinafter, called downlink data) that is communicated from the receiver to the transmitters; and data in the uplink direction (hereinafter, called uplink data) that is communicated from the transmitters to the receiver. The downlink data and the uplink data is respectively transmitted via a downlink-direction optical data communication channel and an uplink-direction optical data communication channel that are multiplexed in the same optical fiber link using the wavelength division multiplex (WDM) technology.
The downlink data is broadcast from the receiver to a plurality of transmitters via the downlink-direction optical data communication channel. The downlink data contains a logical link IDs (LLID) that represents the ID of the destination transmitter. Hence, each transmitter that receives the downlink data refers to the LLID and determines whether the received data is meant for itself. As far as the uplink data is concerned, each transmitter transmits uplink data according to a timeslot (for example, a transmission start timing and a transmittable time period) that is assigned thereto from the receiver. As a result, each set of uplink data gets transmitted to the receiver in a time-shared manner without any collision in the optical fiber link. Moreover, in a PON, in the case in which the receiver assigns a timeslot to each transmitter, instead of implementing fixed bandwidth allocation (FBA) in which the allocation is done in a fixed manner, dynamic bandwidth allocation (DBA) is introduced in which the timeslots are allocated in a dynamic manner depending on the traffic of the uplink data transmitted by each transmitter. As a result, efficient optical data communication is achieved in the uplink direction.
In a QAN too, each transmitter transmits photons according to the timeslot assigned thereto from the receiver. However, unlike in a PON, dynamic allocation of timeslots has not been achieved in a QAN. Moreover, efficiency in sharing the cryptographic keys and using the cryptographic keys has also not been achieved. In a QAN, the timing of use and the amount of use of a shared cryptographic key differs according to the applications executed in the transmitters and the receiver. Hence, in the method in which fixed timeslots are assigned to a plurality of transmitters and photon transmission is done according to those timeslots, it is not possible to achieve efficiency in sharing the cryptographic keys and using the cryptographic keys.